The present invention concerns a device for replacing vertebrae from the spine, and in particular to a length adjustable prosthesis for replacing vertebrae.
If a vertebra becomes diseased or suffers damage it must be removed from the spinal column. Some spacer implants for replacement of defective vertebra are known from the related art. The implants generally have mutually displaceable components, a mechanism to adjust the length of the implant, and special end plates that serve to anchor the implant into adjoining and intact vertebrae.
Such implants are known, for example, from U.S. Pat. No. 4,554,914 to Kapp et al., which discloses a vertebral prosthesis having a pair of support elements and fasteners. The pair of support elements are length adjustable and are inserted into a cavity formed by the removal of a defective vertebra. An expansion device is inserted between opposing surfaces of the adjacent intact vertebrae to force apart the adjacent vertebrae, until the normal height of the vertebra to be replaced is restored. The pair of support elements are adjusted to fit the axial gap, and support the adjoining vertebrae. Once set, fluid and hardening material is inserted around the length-adjusted support elements, to embed them and fixate the adjusted length of the prosthesis.
The support elements in Kapp consist of a bushing having a threaded interior surface, and screws fitting into the threads. Moreover, the bushings are conically tipped at the outer end whereas the screws are wedge-shaped at their outer end. Expanding the support elements in the cavity between the vertebrae causes the outer end of bushings and screws to penetrate the adjacent vertebrae. The conical tips allow rotating the bushing, while the outer wedge-shaped screw ends preclude the screws from rotating, and accordingly length adjustment of the support element can be carried out by rotating the bushing. The fasteners consist of two elongated plates having recesses and screw-holes. These plates are placed on both sides of the spinal column against the dorsal vertebral eminences and screwed into them. The screw holes in the plates are configured in such manner that the screws pass through the dorsal vertebral eminences and as a result fixate the spinal column in the region of the vertebral prosthesis. This known vertebral prosthesis suffers from the drawback that the support elements provide adequate stability only in the case of precise axial loading. Furthermore the insertion of two such support elements into the cavity between the adjoining healthy vertebrae and their length adjustment using the threads is time consuming.
Another device for the prosthetic replacement of a vertebra is known from German Patent DE 30 23 942 to Keller. This known device comprises two members each having a support end for setting up against the adjacent vertebral body and being coaxially telescoping. The telescoping sections of these members comprise a rod at the first member and a complementary bore at the other member. The rod and the bore are provided with an oval cross-section that allows axial displacement of the rod within the bore, if the longer axes of the oval cross-sections are in alignment. If the two members are rotated about the longitudinal axis by 90xc2x0 relative to each other, the threads at the rod engage with the threads in the bore to fix axial movement between the two members. A disadvantage of this known device is that both members must be rotated relative to each other together with the plates that serve as support ends. Thus, the support ends which are in contact with the adjacent vertebrae may damage the healthy vertebrae as the plate is rotated.
Therefore, a need exists for a stable, length-adjustable vertebral prosthesis, which is easily handled in surgery and which restores and maintains the biomechanical and physiological properties of the spinal column despite the removal of one or more defective vertebrae.
The present invention is directed to a vertebral bone prosthetic device comprising an interior body, provided with an outer surface and a first coupling element thereon, and an exterior hollow body with a bore therein extending along a central axis, with the interior body being configured and dimensioned to be slidably received by the exterior body along the central axis, and provided with an interior surface and an exterior surface, the interior surface having a groove. The device also comprises a fixation ring having inner and outer surfaces, with the inner surface having a second coupling element thereon. The fixation ring is configured and dimensioned to be received within the groove of the exterior body for rotational movement about the central axis of the exterior body. Rotation of the fixation ring results in engagement of the first and second coupling elements to thereby prevent relative sliding movement between the interior and exterior bodies. Also, the fixation ring is rotatable between first second positions, the first position disengaging the coupling elements and allowing relative sliding movement and the second position engaging the coupling elements and blocking relative sliding movement.
In an exemplary embodiment, the invention comprises two hollow cylinders, which can be mutually displaced and telescoped into each along a central axis, and a hollow cylindrical affixation ring which is mounted concentrically to the central axis of the two hollow cylinders. The affixation is ring supported in a groove on the inside surface of the outer hollow cylinder such that the affixation ring is free to rotate about the central axis. Sectorial elevations are present on the outside surface of the inner hollow cylinder, in the form of a pitchless thread, and sectorial recesses matching the sectorial elevations are present within the bore of the affixation ring. Engagement of the sectorial elevations and recesses provides axial locking of the two hollow cylinders to one another.
In a preferred embodiment, the diameter of the hollow inner ring and the affixation ring are such as to allow mutual axial displacement of the inner and outer hollow cylinders. In addition, the affixation ring has three sectors having recesses, and the inner hollow cylinder has three sectors having elevations, with each sector subtends an angle of 60xc2x0. Other sectors, also subtending an angle of 60xc2x0 are situated between the sectors fitted with elevations and recesses. Within the other sectors, elevations are absent from the hollow inner cylinder, and recesses are absent from the affixation ring. The elevations and recesses may be engaged in a first angular position (position A), wherein the position of the affixation ring and the inner hollow cylinder are mutually locked, in the direction of the central axis. In a second angular position (position B), the elevations and recesses of the inner hollow cylinder and the affixation ring, respectively, are disengaged, to produce two hollow bodies which are mutually and freely displaceable, in the direction of the central axis. End plates are mounted to the free ends of the inner and outer hollow cylinders. The end plates have a diameter larger than that of the inner and of the outer hollow cylinder. The free surfaces of these end plates will rest against the adjacent vertebrae. The free surfaces of said end plates are further fitted with three-dimensional structures consisting of conical or pyramidal tips. The tips dig into the surface of the adjacent vertebrae, when the present invention is inserted within the evacuated vertebral space and spread apart to secure the prosthesis.
Both hollow cylinders maybe crossed end to end along the central axis by a bore. The cavity, so subtended, may be filled with bone chips to enhance osteointegration of the two adjacent vertebrae to the present invention. To enhance growth of adjacent vertebra onto the implant, the end plates of the present invention which adjoin the vertebra may be fitted with perforations. Inside the outer hollow cylinder a channel in the bore runs in the direction of the central axis. A beak or tab on the inner hollow cylinder engages the channel to secure the two hollow cylinders against rotation. In this manner, the hollow cylinders are precluded from mutual rotation when the affixation ring is rotated.
An offset maybe present on the outside surface of the affixation ring, which can be displaced in such manner inside a corresponding clearance to allow rotation of the ring between position A and position B. Further the beak of the affixation ring when in position A rests against one sidewall of the clearance and when in position B rests against the other sidewall. The lateral stops, so implemented for the offset at the affixation ring, allow for easily ascertaining the first angular position of the affixation ring, in which both hollow cylinders are axially locked in position, or the second angular position of the affixation ring, wherein the two hollow cylinders are mutually axially displaceable. To secure the affixation ring in a selected position, an axially projecting, V-shaped elevation may be present at the offset, which detents into corresponding flutings in the clearance, whereby the affixation ring is detachably locked in positions A or B.
Essentially, the general advantage offered by the present invention include high stability, which is achieved under axial load, and that the adjacent vertebrae may knit over an adequately large cross-section of the prosthesis. Additionally, when the present invention is inserted into the evacuated vertebral space, the device is extended by expansion tongs until the end plates come to rest against the adjoining, healthy vertebrae, and the tips on the end plates penetrate sufficiently deeply into these vertebrae. For that purpose, the affixation ring can be rotated, beforehand, into the second angular position (i.e., position B). Once the implant has been adjusted to the required length, the affixation ring is rotated into its first angular position (position A) by a bar, which is insertable into a bore in the affixation ring especially provided for that purpose. In this manner, the prosthesis device is locked at the desired length. This lengthwise locking of the implant assures easy handling during the device implantation.